Antenna with conical transmission line feed

ABSTRACT

The apparatus of the present invention provides an antenna with a dual circularly polarized monopulse feed system which employs a conical or angular transmission line to guide a transverse electromagnetic spherical wave to a reflector. For an n element conical transmission line feed where n is a positive integer no less than three, normal modes with phase progressions or delays of 360* /n from one element to the next are used to illuminate the reflector for rotationally symmetric sum patterns with either sense of circular polarization. Similarly, for an n element conical transmission line feed where n is a positive integer no less than five, normal modes with phase progressions or delays of 720* /n from one element to the next are used to illuminate the reflector for rotationally symmetric difference patterns with either sense of circular polarization.

United States Patent FEED 12 Claims, 5 Drawing Figs.

US. Cl 343/777, 343/778, 343/779, 343/840 Int. Cl H01q 13/00, H01q 19/12 Field of Search 343/777,

[56] References Cited UNITED STATES PATENTS 2,918,673 12/1959 Lewis etal i. 343/779X Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorneys-James K. Haskell and Robert H. Himes ABSTRACT: The apparatus of the present invention provides an 'antenna with a dual circularly polarized monopulse feed system which employs a conical or angular transmission line to guide a transverse electromagnetic spherical wave to a reflector. For an n element conical transmission line feed where n is a positive integer no less than three, normal modes with phase progressions or delays of 360ln from one element to the next are used to illuminate the reflector for rotationally symmetric sum patterns with either sense of circular polarization. Similarly, for an n element conical transmission line feed where n is a positive integer no less than five, normal modes with phase progressions or delays of 720ln from one element to the next are used to illuminate the reflector for rotationally symmetric difference patterns with either sense of circular polarization.

Patented April 6, 1971 3,573,836

5 Sheets-Sheet l Ana/W042i.

024M: 5. 4070444, ZA VMO/VO A4 aw/M14, 5

PM M

Afro/awn Patented A ril 6, 1971 3,573,836

5 Sheets-Sheet 2 Patented A ril 6, 1971 5 Sheets-Sheet 5 Patented April '6, 1971 5 Sheets-Sheet 5 YNQkKNk 1 ANTENNA WI'I'IICONICAL TRANSMISSION LINE FEED BACKGROUND OF THE INVENTION Conventional feeds consist of a relatively small antenna such as, for example, a dipole, slot, horn, spiral or log-periodic antenna which illuminates a reflector or lens with a spherical wave. The reflector converts the radiated spherical wave into a plane wave for maximum gain or possibly into a different spherical wave if a Cassegrain .subreflector is used. In the case of an antenna designed on the four-arm equiangular spiral concept, radiation is inherently circularly polarized.'l-lowever, to obtain both senses of circular polarization, two spiral anten- .nas are required, one with arms spiraling in the clockwise direction and the second with arms spiraling in a counterclockwise direction. Furthermore, the relative phase between sum and difference channels varies with frequency for the spiral antennas.

In another concept, a conical array of eight linearly polarized log-periodic elements provides sum and difference patterns for both senses of circular polarization simultaneously. Because the relative phase between sum and difference beams is independent of frequency, tracking information is easily obtained. It is difiicult, however, to achieve satisfactory impedance or pattern characteristics of the log-periodic parasitic monopulse elements. In addition, the phase center movement of even a successful log-periodic antenna limits the performance of a constant beam width system. The conical transmission line feed system of the present invention is simpler, less expensive and has no phase center movement.

SUMMARY OF THE INVENTION In accordance with the present invention, a feed system is implemented utilizing two or more angular or conical struts emanating from, but not contacting, a small spherical ball at the focal point of a parabolic reflector or the focal point of an ellipsoidal reflector nearest the surface thereof to equallyspaced points equidistant from the center of the reflector. The struts are excited by a corresponding number of coaxial lines which by way of example may be disposed along the respective struts to the vertex where the center conductors of the coaxial lines are connected to the small spherical ball. Alternatively, the coaxial lines may be disposed along the axis of the reflector or come from behind the focal point. The struts are spaced such that the transmission line system is rotationally symmetric. A two-conductor feed may be used to obtain a linearly polarized sum pattern. A three-conductor feed can provide two sum beams with opposite circular polarizations. A four-conductor feed can provide two circularly polarized sum beams and a degraded difference pattern. Five or more conductors are required for two circularly polarized sum and difference beams (four beams in all).

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a front perspective view of the conical transmision line feed with a parabolic reflector;

FIG. 2 illustrates a closeup view of feed point of conical transmission line feed of FIG. 1;

FIG. 3 illustrates a view of the antenna system of FIG. 1 with a block diagram of a feed network;

FIG. 4 illustrates a block diagram of the six port feed network of the apparatusof FIG. 3; and

FIG. 5 illustrates a view of the disclosed antenna system employing an ellipsoidal reflector.

Referring now to FIGS. 1-3 of the drawings, there is shown the antenna system of the present invention which includes a parabolic reflector with, by way of example, six support struts 11, I2, 13, 14, 15, 16, disposed at equal intervals about the outer periphery of reflector 10 and extending to respective points just short of the focal point of the parabolic reflector 10. Alternatively, struts 11-16 may extend from equally spaced points equidistant from the center of parabolic reflector 10. Input to theantenna includes coaxial lines 18. 19, 20, 21, 22, 23, disposed along the support struts II to 16, respectively, on the sides opposite from the reflector 10. In each case, the outer conductor of the coaxial lines 18 to 23 is terminated at the extremity of the strut 11-16, respectively, and the center conductor extended to a small conductive sphere .17. Conductive sphere 17 is required only if the feedpoint region is an appreciable fraction of a wavelength. The feedpoint region encompasses the openings of the outer conductors of the coaxial lines 18-23. This is more clearly illustrated in FIG. 2 of the drawings. As previously specified, coaxial lines 18- 23 may be disposed along the axis of parabolic reflector 10, or alternatively, enter from behind the focal point thereof.

Referring particularly to FIG. 3, the coaxial lines 18-23 from the reflector 10 constitute inputs to the antenna system and are designated T.-T,,, respectively. The terminals T,T,, connect to a six-port feed network 25 which has 2 (summation) terminals 30, 31 and A (difference) terminals 32, 33. The summation terminals 30, 31 feed the reflector 10 with progressively increasing or decreasing phase of 60 to produce single beams of right-circular and left-circular polarization. The difl'erence terminals 32, 33, on the other hand, feed the reflector 10 with progressively increasing or decreasing phase of which accumulates to 720 in one revolution to generate dual beams of righbcircular and left-circular polarization for tracking purposes. The rightand left-circular-polarized sum beams may be operated simultaneously to generate a linear-polarized beam that is poled in any desired direction.

Referring to FIG. 4, there is shown a schematic block diagram of the six-port feed network 25 of FIG. 3. The six-port feed network 25 constitutes an empirical combination of quadrature hybrid and tapered-line magicT networks. In FIG. 4 a convention is used in connection with the respective quadrature hybrids wherein the feed side comes out in phase quadrature with the nonfeed side and the label H refers to a 3 db. quadrature hybrid wherein the input power is divided equally between the two output terminals. In addition, a convention is used in connection with tapered-line magic-T's wherein one side is defined as the 2 side. The label T refers to a 3 db. tapered-line magic-T for which the coupled outputs are split in power by the ratio 1:1 and the label T refers to a 4.8 db. tapered-line magic-T for which the coupled outputs are split in power by the ratio of 2:1 with the greater power output being on the 2 side. When the feed is applied to the 2 side, the voltages appearing at the outputs are in phase and when the feed is applied to the non-2 side the voltages appearing at the outputs are out of phase.

Referring now to FIG. 4, the six-port feed network 25 includes Z (summation) terminals 30, 31 and A (difference) terminals 32, 33 and output terminals 34-39 which connect, respectively, to terminals T,T The feed network 25 includes 3 db. tapered-line magic-T's 40, 41 which have Z-side outputs connected to terminals 35, 38 and non-E side outputs connected to terminals 36, 39, respectively. In addition, network 25 includes 4.8 db. tapered-line magic-T's 42, 43 which have Z-side outputs connected to temtinals 34, 37, respectively, non-E side outputs connected to the Z-side inputs of tapered-line magic-T's 41, 40, respectively, and the E-side inputs terminated with impedances 44, 45, respectively. An additional 3 db. quadrature hybrid 46 has an output 47 connected to the non-Z side input of 4.8 db. tapered-line magic-T 42 and an output 48 connected to the non-2 side input of 3 db.

tapered-line magic-T 41. Quadrature hybrid 46 has inputs 49, t

50 opposite outputs 47, 48, respectively. Similarly, an additional 3 db. quadrature hybrid 51 has an output 52 connected to the non-2 side input of tapered-line magic-T 40 and an output 53 connected to the non-2 side input of tapered-line magic-T 43. Quadrature hybrid 51 has inputs 54, 55 opposite the outputs 52, 53, respectively. Lastly, 3 db. tapered-line magic-T's 60, 61 have Z-side outputs connected to inputs 50, 55 and non-Z side outputs connected to inputs 54, 49, respectively, of quadrature hybrids 46 and 51. The E-side inputs of tapered-line magic-T's 60, 61 are connected to A-terminals 32, 33, respectively, and the non-2 side inputs to E-terminals 31, 30, respectively.

In operation, a signal applied to E-terrninal 30 corresponds to a sum pattern excitation and produces progressive phases at the terminals T,T of 60. A signal applied to E-terminal 31, on the other hand, produces a sum pattern of the opposite circular polarization of phase progression of 60 at the terminals T,T Likewise, a signal applied to A-terminals 32 or 33 produces difference patterns with phase progressions of +120 or l20, respectively, at the terminals T,-T,,. The energy available at the terminals 3439 progresses through the coaxial lines 18-23 and along the center conductors thereof to the spherical ball 17 where it is reflected to produce a transverse electromagnetic wave along the struts 11-16 which form a conical transmission line. The constant phase surfaces for the electric and magnetic fields are spheres centered at the sphere 17. Since the phase fronts for the transmission line wave are spherical, the parabolic reflector 10 will tend to reflect this wave as a plane wave. Although the reflected wave tends to be scattered somewhat by the presence of the struts 11-16, very little of the energy should be reflected into the feed terminals T --T Sum patterns produce a single main beam and difference patterns produce rotationally symmetric difference beams with a point null.

Referring to FIG. 5, there is shown the antenna system of FIG. 1 with an ellipsoidal reflector 70 substituted for the parabolic reflector 10. In this case, the small conductive sphere 17 is located at the focal point of ellipsoidal reflector 70 nearest to the surface thereof. Operation is the same as for the antenna system of FIGS. l3.

We claim:

1. A broadband antenna system comprising a convex reflector having an axis of symmetry and a focal point therealong;

means for providing a predetermined number of longitudinal conductive surfaces from equally-spaced points on said reflector equidistant from the center thereof substantially to said focal point; and

a number of coaxial lines equal to said predetermined number having center conductors connected together at said focal point thereby to provide an input to said anten- 2. The broadband antenna system as defined in claim I wherein said convex reflector has a parabolic configuration.

3. The broadband antenna system as defined in claim 1 wherein said convex reflector has an ellipsoidal configuration and said focal pointis the nearest focal point from the surface of said reflector.

4. The broadband antenna system as defined in claim 1, additionally including a small conductive sphere at the junction of said center conductor of said coaxial lines.

5. A broadband antenna system comprising:

a convex reflector having an axis of symmetry and a focal point therealong;

n equal-length coaxial lines extending from equally-spaced points about the periphery of said convex reflector to said focal point where n is a positive integer no less than three;

a conductive sphere disposed at said focal point and connected to the respective center conductors of said n coaxial lines;

means for providing n longitudinal conductive surfaces from said equally-spaced points about the periphery of said convex reflector along the outer conductors of said n coaxial lines; and

means for feeding said n coaxial lines with signals having substantially equal phase differences from one coaxial line to the next adjacent coaxial line of said n coaxial lines.

6. The broadband antenna system as defined in claim 5 wherein said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase progression of 360/n.

7. The broadband antenna system as defined in claim 5 wherein said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase dela of 360/n.

8. The roadband antenna system as defined in claim 5 wherein n is a positive integer no less than five and said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase progression of 720/n 9. The broadband antenna system as defined in claim 5 wherein n is a positive integer no less than five and said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase delay of 720ln.

10. A broadband antenna system comprising:

a parabolic reflector having a focal point;

six equal-length coaxial lines extending from equally-spaced points about the periphery of said parabolic reflector to said focal point;

a conductive sphere disposed at said focal point and connected to the respective center conductors of said six coaxial lines;

six conductive struts disposed from said equally-spaced points about the periphery of said parabolic reflector along the outer conductor of said coaxial lines; and

means including quadrature hybrids and tapered-line magic-T's connected to said six coaxial lines for feeding said coaxial lines with signals having substantially equal phase differences from one coaxial line to the next adjacent coaxial line.

ll. A broadband antenna system comprising:

a convex reflector having an axis of symmetry and a focal point therealong;

n equal-length coaxial lines extending from equally-spaced points about the periphery of said convex reflector to said focal point where n is a positive integer no less than three;

means for providing a conical transmission line from said focal point to said convex reflector; and

means for feeding said n coaxial lines with signals having substantially equal phase differences from one coaxial line to the next adjacent coaxial line of said n coaxial lines.

12. The broadband antenna system as defined in claim 11 additionally including a conductive sphere disposed at said focal point and connected to the respective center conductors of said n coaxial lines. 

1. A broadband antenna system comprising a convex reflector having an axIs of symmetry and a focal point therealong; means for providing a predetermined number of longitudinal conductive surfaces from equally-spaced points on said reflector equidistant from the center thereof substantially to said focal point; and a number of coaxial lines equal to said predetermined number having center conductors connected together at said focal point thereby to provide an input to said antenna.
 2. The broadband antenna system as defined in claim 1 wherein said convex reflector has a parabolic configuration.
 3. The broadband antenna system as defined in claim 1 wherein said convex reflector has an ellipsoidal configuration and said focal point is the nearest focal point from the surface of said reflector.
 4. The broadband antenna system as defined in claim 1, additionally including a small conductive sphere at the junction of said center conductor of said coaxial lines.
 5. A broadband antenna system comprising: a convex reflector having an axis of symmetry and a focal point therealong; n equal-length coaxial lines extending from equally-spaced points about the periphery of said convex reflector to said focal point where n is a positive integer no less than three; a conductive sphere disposed at said focal point and connected to the respective center conductors of said n coaxial lines; means for providing n longitudinal conductive surfaces from said equally-spaced points about the periphery of said convex reflector along the outer conductors of said n coaxial lines; and means for feeding said n coaxial lines with signals having substantially equal phase differences from one coaxial line to the next adjacent coaxial line of said n coaxial lines.
 6. The broadband antenna system as defined in claim 5 wherein said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase progression of 360*/n.
 7. The broadband antenna system as defined in claim 5 wherein said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase delay of 360*/n.
 8. The broadband antenna system as defined in claim 5 wherein n is a positive integer no less than five and said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase progression of 720*/n.
 9. The broadband antenna system as defined in claim 5 wherein n is a positive integer no less than five and said substantially equal phase differences from one coaxial line to the next adjacent coaxial line constitutes a phase delay of 720*/n.
 10. A broadband antenna system comprising: a parabolic reflector having a focal point; six equal-length coaxial lines extending from equally-spaced points about the periphery of said parabolic reflector to said focal point; a conductive sphere disposed at said focal point and connected to the respective center conductors of said six coaxial lines; six conductive struts disposed from said equally-spaced points about the periphery of said parabolic reflector along the outer conductor of said coaxial lines; and means including quadrature hybrids and tapered-line magic-T''s connected to said six coaxial lines for feeding said coaxial lines with signals having substantially equal phase differences from one coaxial line to the next adjacent coaxial line.
 11. A broadband antenna system comprising: a convex reflector having an axis of symmetry and a focal point therealong; n equal-length coaxial lines extending from equally-spaced points about the periphery of said convex reflector to said focal point where n is a positive integer no less than three; means for providing a conical transmission line from said focal point to said convex reflector; and means for feeding said n coaxial lines with Signals having substantially equal phase differences from one coaxial line to the next adjacent coaxial line of said n coaxial lines.
 12. The broadband antenna system as defined in claim 11 additionally including a conductive sphere disposed at said focal point and connected to the respective center conductors of said n coaxial lines. 